HOTSPOTS AND PLATE MOTION(Exercise courtesy of Amherst College Geology Department)

NOTE: this homework may be done in groups of up to three people.

At a divergent plate boundary - or "spreading center"
- asthenospheric mantle rises in a long linear belt along the plate boundary.
In contrast, a hotspot is a point source of upwelling mantle, much like
a fountain. Well known, presently active hotspots include Hawaii, Yellowstone,
and Iceland. There are many other, less familiar hotspots, however, like
the Marquesas and Austral hotspots in the Pacific. In all, there are 41
hotspots active today. Like the rising arms of convection cells, hotspot
mantle decompresses as it ascends - undergoing partial melting and yielding
basaltic magma to the surface.

If we examine Hawaii, as a typical hotspot, we see that a long chain
of islands and seamounts (submerged oceanic peaks) trails off from the
"Big Island" to the northwest, bending to a more northerly trend
at Yuryaku seamount. Only the island of Hawaii is presently volcanically
active; all the other islands are inactive. Each island was built from
basaltic volcanism derived from the hotspot plume. The chain-like array
of these islands results from the motion of the Pacific plate over the
hotspot's point source of upwelling mantle; the plate motion carries each
newly built island away from the hotspot over time. Hotspot island chains,
therefore, preserve a record of the motion of the plate on which they
are built.

THE HOTSPOT FRAME OF REFERENCE:

Look at the map below of the Hawaii Island-Emperor Seamount chain, the
Line and Marquesas Islands chain, and the Marshall and Austral Islands
chain in the Pacific. All three are presently active hotspot chains, and
have been active for at least 60 Ma, based on the age of basalt flows
in the island chains.

1) If you assume that there is a hotspot under each of these tracks, what can we hypothesize about the motion of the hotspots relative
to each other (2 points)?

2) What can we infer from this about the rigidity of the Pacific plate (2 points)?

THE MOTION OF THE PACIFIC PLATE:

Evidence like that of the Marquesas, Austral, and Hawaiian hotspots
demonstrates that hotspots stay fixed relative to each other for long
periods of geologic time. This provides a frame of reference within
which to analyze the motion of plates. In this exercise, you will use
the Hawaiian hotspot to examine the motion of the Pacific plate over
the time span during which that hotspot has been active.

Examine the following table. It shows the age of basaltic volcanism
on each island in the Hawaiian-Emperor chain, the distance of the island
from the active hotspot measured along the chain, and the elevation
of the island or seamount above or below sea level, respectively.

Age (Ma)

Distance from Hawaii (km)

Hawaii

0

0

Maui

0.8

221

Lanai

1.3

226

Molokai

1.8

270

Niihoa

4.9

565

Kauai

5.1

519

Necker

10.3

1058

French Frigate shoal

12

1209

Pearl reef

20.6

2281

Midway

27.2

2432

Daikokuji

42.2

3493

Yuryaku

43.4

3520

Koko

48.1

3758

Ojin

55.2

4102

Jingu

55.4

4175

Nintoku

56.2

4452

Sulko

59.6

4860

3) How long has the Hawaiian-Emperor hot spot been active (2 points)?

Now, graph the distance of each island or seamount from Hawaii (vertical
axis) versus the age of the island (horizontal axis). You can do this
with a computer spreadsheet program if you wish. This is a distance/time
graph and so the slope of any line on this graph represents velocity. Print out your graph and hand it in (4 points).

4) Is the plate velocity constant over time (in other words, do these points define a straight line), or does it change (and if so, when)? (2 points)

5) In the map above, the first number is the age of the island or seamount (if you print out the graph, it becomes a bit easier to read). Consider these data along with the distance/time graph you have made. Did the direction of the plate change at the same time as the speed changed? (2 points)

ELEVATION OF THE HAWAIIAN ISLANDS:

The second number in the figure above is the depth in meters below sea level. Now plot a graph of the age of each island in the Hawaiian-Emperor chain
(horizontal axis) versus the elevation of each island above or below
sea level (vertical axis). Draw a single best fit curve through these
points. (4 points)

6) What can you say about the elevation of hotspot-produced islands
as they move farther and farther away from the site of mantle upwelling? (2 points)

7) Can you think of an explanation for this relationship? (2 points)

8) The volcanic region around the Tharsis Bulge
on Mars is the tallest in the solar system. Why do huge volcanoes form
on Mars, and not on Earth? (2 points)